Digital transmission systems in which speech information is transmitted apply speech coding to convert analog speech into digital form. For example, in a fixed telephone network, analog speech is converted into a digital signal having the bit rate of 64 kbps. In several transmission systems, such as cellular radio systems, the aforementioned transmission rate is impractical since it takes up too much of the transmission capacity. There have been attempts to implement the speech coding used in these systems with as small a transmission band and as low a bit rate as possible.
The transmission band and the bit rate can be reduced by processing digitalized speech with the methods of signal processing, for example, by decreasing redundant information and by compressing speech. At the same time, as these signal processing methods reduce the transmission band required, the quality of the transmitted speech deteriorates. This deterioration is especially detrimental when the process of coding/decoding is performed several times over the connection, as, for example, in a cellular network, when the call is between two terminal equipment.
Several speech coding methods have been developed, and the aim of all of the methods is to obtain a bit rate that is as low as possible without any deterioration in the speech quality. A mean opinion score (MOS), based on listening tests, is generally used as a criterion for speech quality. The table of FIG. 1 shows the MOSs of several known speech coding methods in the case in which the connection has one transcoder and the case in which the connection has two transcoders as when the connection is between two terminal equipments in a cellular radio system. As it can be seen, with two coders, the quality of speech generally deteriorates. This phenomenon is known and there have been attempts to take account for it in the development of speech coding methods, but, even in the latest coders, speech quality is not good when two coders are used over the same connection.
A known arrangement aiming at eliminating the aforementioned problem is disclosed in U.S. Pat. No. 4,924,480. As mentioned above, analog speech is converted into a digital format comprising a 64 kbps signal. This signal is presented in the form of an 8-bit A-law word. In the method disclosed in the reference, the least significant bit (LSB) of this 8-bit word is used so that a coder may manifest its existence to another coder by this bit. In practice, the coder of the transmitter may thus transmit its own synchronization pattern, which is detected by the coder of receiver, in the place of this LSB. When the coders have detected each other's existence, the coders switch to a so-called padding stripping mode in which the coders do not decode the signal for the transmission, but only adapt its transmission rate for the transmission. Successive coding/decoding operations can therefore be avoided. The problem with the disclosed method is that the LSB of each 8-bit A-law PCM word is used for purposes other than speech transmission, and the quality of the transmitted speech therefore deteriorates. Furthermore, the required handshaking procedure between the coders is slow, since each bit in the synchronization pattern must be repeated six times, and each synchronization pattern must be sufficiently long to enable reliable recognition. Also, each synchronization pattern must be repeated at least once.